This invention pertains to means for measuring the fluid pressure of plastic in a mold cavity. In particular, the axial force exerted on a mold ejector pin or dummy pin by plastic in a mold cavity is sensed with an electro-mechanical sensor or transducer. Piezo-electric and electro-mechanical transducers housed in steel bodies are available for the measurement of these forces.
In a typical application, the sensor is positioned in a slot milled into the mold ejector plate such that the center of the sensing element is aligned with the center of the ejector pin or dummy pin. The lead wires from the sensor are brought out of the mold through a channel in the mold ejector plate. In cases where the lead wire channel cannot be straight, or the channel must be smaller than the width of the sensor, a peanut shaped or a button mold pressure sensor may be used.
A generally peanut shaped mold pressure sensor as disclosed in U.S. Pat. No. 4,760,749 may be used where a constant radius full width slot is required. The constant radius full width slot allows the sensor to be inserted and removed from the ejector plate without disassembly of the mold or ejector plate. Alternatively, an oblong pocket and a smaller channel in the mold ejector plate may be used. Use of the smaller channel with the peanut shaped sensor eliminates the option of inserting and removing the sensor without mold disassembly.
Button mold pressure sensors are characterized by T-412 and T-413 sensors supplied by RJG Technologies, Inc., Traverse City, Mich. Button mold pressure sensors are typically installed in a pocket under a mold ejector pin and cannot be removed without disassembling the mold ejector plate and ejector retainer plate.
Where a straight full width channel extends to the edge of the ejector plate, an oblong slide mold pressure sensor can be inserted or removed from the mold without dismantling the mold or ejector plate. Slide mold pressure sensors are characterized by the D-M-E Standard SS-405C and SS-406C sensors from D-M-E Company, Madison Heights, Mich. Slide sensors are generally rectangular in shape and have a Teflon or polyvinyl chloride coated cable enclosed in a flexible armored jacket leading from the sensor.
The cable and armored jacket leading from the sensor are not structurally rigid and the user must provide a means to retain the slide sensor in the ejector plate channel. A common method for retaining the slide sensor is with a friction spring attached to the top surface of the sensor. The friction spring is not always sufficient to retain the sensing element in the desired location.
The sensing element of the slide sensor is located by the side walls of the channel, and the end of the channel. Two standard configurations exist for the slide sensor. The difference between the two configurations is that the sensor body is one-eighth inch longer from the sensing element to the tip end of the sensor body. The two configurations are not easily interchangeable in the field. Additionally, a slot that is milled too long will incorrectly locate the sensor element of either transducer. In such a situation, the transducer is spaced from the end of the slot with an insert, or the ejector plate is reworked.